This invention relates to a sensor for measuring the electrical conductivity of a liquid. More specifically, the present invention provides a conductivity sensor with an integrated temperature sensor for measuring ion concentrations in very small samples, including in vivo and in-situ chemical or biological fluid analysis.
Conductivity of a liquid solution provides information about the solution. Measuring the conductivity of a solution by electrical means is a difficult problem, especially in small volumes. Electrons and ions accumulate at an electrode/liquid interface of an electrical sensor, creating a surface potential between the electrode and the solution that must be accounted for in determining conductivity.
Many devices might benefit from a conductivity sensor having accuracy in small liquid volumes. For example, development of a portable unit for purifying water in remote areas requires use of small sensors to detect the presence of salts, or other solutes, to measure effectiveness of the unit. Accurate small sensors might also be advantageously applied as part of a system to automatically determine rinse cycle end in a clothes washer, cycle end in a water softener, or a cycle end in a water purifier.
Commercially available conductivity probes have diameters greater than 1 cm, limiting their potential applications. Such a probe is accurate for sample volumes of more than 10 ml. Their size leads to a large time constant, requiring tens of seconds to reach equilibrium, thus yielding a slow response time. As probes become smaller, sample size and response times become smaller but surface effects and other problems become more important. Small probes also present fabrication problems unresolved by the conventional configuration of larger probes, i.e., the large probes do not scale down.
An accurate sensor must account for the variance of conductivity with temperature. In large probes, a separate temperature sensor can be used to measure the temperature and adjust the conductivity measure. Use of a separate temperature probe is inaccurate or impossible with a small probe in a small liquid volume, where the local temperature at the probe is influential.
In certain settings, the ambient temperature can be controlled such that it is known with a high degree of accuracy. However, conventionally sized and packaged conductivity probes are inaccurate for small sample volumes, even if the temperature is known to high accuracy. As the volume of liquid approaches the size of the sensor, large errors result in conductivity measurements, since the cell constant, Kc, changes, i.e., it is no longer a constant. Kc is important since it is used to calibrate the sensor, in order to determine conductivity, and therefore ion concentration, from the electrical resistance.
The present invention provides a microscale thin film liquid conductivity sensor capable of sensing conductivity in very small volumes and which can be fabricated as part of an integrated circuit. A preferred embodiment includes an integrated temperature sensor.
The thin film sensor is formed on a suitable surface. The surface is preferably an insulator, but may be conducting, if the electrodes are on an insulating surface. A preferred embodiment insulates the electrodes by use of a dielectric layer that is deposited on top of a silicon wafer substrate. A sensor tip is integrated on the top surface of the substrate. The substrate is fabricated into a sensor shape with a small sharp tip at one end and an opposite end that can be larger to accommodate electrode measurement pads. The larger end might also be used for integration of measurement circuits or accommodating wire bonding pads. In a preferred embodiment, the tip also accommodates an integrated temperature sensor to enable local temperature measurements. The sensor is a thin-film resistor preferably enclosed within a layer of the sensor tip and having a serpentine shape to obtain a desired resistance in a smaller active area while consuming a small area of the sensor tip.
The sensor of the invention may be made very small and may be integrated with circuitry. An exemplary 100 xcexcm sensor has been fabricated. Larger sensors of the invention can be made arbitrarily smaller than commercially available 1 cm diameter sensors. It is the structure of the invention which is important, however, allowing not only very small sensor sizes but also permitting integration of the sensor with integrated calculation circuits, etc. Artisans will appreciate the size advantages offered by the structure, while recognizing that size may be optimized to achieve desirable goals. For example, smaller sensors of the invention tending toward 100 xcexcm and below are more difficult to fabricate, and more difficult to obtain an accurate measurement than a 500 xcexcm sensor or 1 mm sensor of the invention. However, the smaller sensors obviously consume a smaller area of substrate. These competing design goals may be balanced to suit particular applications in using a sensor of the invention